1. Field of the Invention
The present invention pertains to valves. More specifically, the present invention pertains to low pressure relief valves. In particular, the present invention pertains to low pressure relief valves suitable for maintaining containment vessels or systems at very low positive pressures or negative pressures (vacuum)
2. Description of the Prior Art
Many containment vessels or systems are designed for low pressure (positive or negative). If the design pressure or vacuum (negative pressure) is exceeded, even momentarily, the vessel may fail resulting in much physical damage and harm to equipment, personnel and the environment. To remedy this situation such containment vessels or systems are commonly provided with some type of relief valve which helps maintain the vessel or system within its design parameters. For example, if a vessel is to be maintained at or below a specified pressure, the valve will open in response to greater pressures to allow fluid flow from the vessel until the pressure is reduced to an acceptable level. If the vessel operates under a vacuum (negative pressure) the relief valve may be designed to open when the negative pressure becomes too great, allowing fluid flow from the exterior of the vessel into the vessel until the negative pressure is reduced to an acceptable level.
Most pressure relief valves are provided with a closure member which is biased against the seat of a valve port to close the valve. The spring is designed so that upon increase of pressure above a specified amount, the closure member will be lifted off of its seat to allow fluid flow through the valve port. Examples of such spring-influenced valve closure members are disclosed in U.S. Pat. Nos. 4,632,146; 4,662,616; 4,765,504; 4,951,701 and 5,213,586. An example of a spring-loaded vacuum (negative pressure) relief valve may be seen in U.S. Pat. No. 4,133,346.
Relief valves for very low positive pressure or vacuum containment vessels need to be very sensitive since a relatively small increase in pressure (positive or negative) may result in failure of the vessel. Spring loaded relief valves are more suitable for higher pressure operation and are frequently not as sensitive as required for low pressure operation. Furthermore, springs, hinged plates or other mechanisms used in biasing closure members against a valve seat are subject to failure or malfunctioning from corrosion and exposure to the contents of the vessel with which it is used or the atmosphere surrounding the vessel.
There are other relief valves or vents in which a closure member, such as a ball, operates without the influence of a spring. Examples of such may be seen in U.S. Pat. Nos. 2,935,990; 3,733,449 and 4,518,363. However, these valves, typically referred to a ball float valves, require rising water or other liquids for movement of a ball away from or toward the seat of a valve port. Such a valve would not be suitable for a containment vessel or apparatus which is filled with a gaseous fluid or a liquid above which a vapor space exists.